This invention relates to a fuel injection apparatus used to inject fuel into a cylinder of a Diesel engine.
A fuel injection apparatus generally comprises a fuel tank, fuel supply pump, fuel filter, fuel injection pump, injection nozzle etc., and is used to inject liquid fuel, under pressure, into a cylinder of a Diesel engine. The fuel supply pump compresses liquid fuel from a fuel tank and sends it to a fuel injection pump. A fuel injection pump usually comprises a cylinder and a plunger and has a suction inlet and an escape hole. The plunger can be rotated relative to the cylinder with its axis as a center. A spiral cutout is formed around the outer peripheral surface of the plunger. The plunger is reciprocally moved in synchronism with the operation of an engine and compresses fuel sent from the fuel supply pump, the fuel is injected from the injection nozzle into the cylinder of the engine. In this way the fuel is supplied to the engine and the injection time and injection amount are controlled according to the operation states of the engine. This is done by rotating the plunger by the use of a rack etc. and varying a positional relation of the spiral cutout to the suction inlet and the escape hole of the cylinder. With this arrangements, the starting and/or ending of fuel injection is likely to be quickened or slowed down. To cope with this situation, an electromagnetic valve is provided, as disclosed in Japanese Patent Disclosure (KOKAI) No. 54-50726, to accurately control the fuel injection time and fuel injection amount when fuel is injected into the engine. If, in this case, electromagnetic pulses to the electromagnetic valve are electrically controlled, the fuel injection time and fuel injection amount are accurately determined, making them commensurate with the operation states of the engine. In this method, however, high-pressure fuel is directly loaded to the electromagnetic valve and it is therefore necessary to provide a value capable of withstanding the high pressure. Even when a high pressure electromagnetic valve is provided, however, the reliability of the valve is detrimentally effected by injecting fuel under high pressure. To overcome this drawback, a method is adopted to provide a restrictor partway of a passage for connecting the electromagnetic valve to the pump chamber, and to, thusly, lower the fuel pressure transmitted to the electromagnetic valve. If such a restrictor is used, however, the "cutting of the fuel injection", i.e. a tendency of the fuel injection amount to be gently decreased at the final phase of fuel injection, fails to take a proper pattern and thus it takes a longer time until the injection of a predetermined amount of fuel is completed. This is the reason why the restrictor cannot be used.
In the above-mentioned method using the electromagnetic valve, an amount of fuel to be injected is determined by the valve opening time, i.e. the fuel injection time, making it necessary to accurately controllably open and close the electromagnetic valve for a very short period of time. Moreover, the electromagnetic valve requires an excellent response characteristic and high finishing accuracy is also required to prevent a variation in the amount of fuel to be injected.